This invention relates to positioning apparatus, and in particular to positioning apparatus suitable for determining the position of a moving vehicle, and to a positioning method.
Tracking of the position of high dynamic moving vehicles whilst in flight is desirable for many reasons, particularly as it can allow the location of a vehicle to be accurately determined even though the location may not be visible from a viewing site.
For this purpose, it is known to use a translator device forming part of the vehicle, and a base or ground station remote from the vehicle. The translator device receives positioning signals from a number of satellites of, for example, the GPS constellation, and amplifies and frequency translates the signals before transmitting them as translator signals to the base station. A positioning receiver in the base station then processes the translator signals to determine position information relating to the translator device and thus the host vehicle. By recording the translator signals, the base station can plot the path of the vehicle accurately and with whatever time intervals are required, after the vehicle""s flight.
Although translator systems are immune to the effects of acceleration bursts, the bandwidth required for transmission of the translator signals is very high, for example, at least 5 Mbits per second needing to be transmitted for GPS satellite signals
It has been proposed, for use in meeting the proposed xe2x80x9cenhancedxe2x80x94911xe2x80x9d US wireless telephone requirement, to include simplified GPS receivers in wireless telephones. Such receivers include circuitry arranged to process positioning signals transmitted by the satellites, to determine interim position information. Extraction of the data carried on the positioning signals is carried out at a base or ground station, which can thereby calculate position information relating to the GPS receiver and thus the wireless telephone.
Such a system is beneficial in that it removes a substantial processing burden from the wireless telephone, yet allows rapid determination of its position. Furthermore, the accuracy of location determination of such a system can easily be increased through the use of differential GPS techniques.
According to this invention, there is provided positioning apparatus for use in a moving vehicle comprising:
a receiver for receiving coded ranging signals from each of a plurality of remote transmitters including at least one positioning satellite;
a code correlator device arranged to correlate a locally provided code with the code contained in modulation of the coded ranging signals, and to provide correlator output signals in response thereto;
a code tracking device responsive to at least some of the correlator output signals to provide code tracking output signals;
a carrier frequency estimator device, arranged to receive at least some of the correlator output signals, to estimate the carrier frequency of the coded ranging signals, and to provide carrier frequency estimation signals in response thereto;
a carrier sampling device arranged to receive and to sample at least some of the correlator output signals to provide carrier sample signals from which information about the phase of a carrier of the coded ranging signals can be determined; and
a transmitter arranged to receive the code tracking output signals, the carrier sample signals and the carrier frequency estimation signals, and to transmit signals which are representative of the code tracking output signals, the carrier sample signals and the carrier frequency estimation signals, to allow position information relating to the apparatus to be determined by further processing at a location remote from the vehicle.
In this way, it is possible to construct positioning apparatus which is capable of providing precision position tracking and which is less susceptible to losing tracking lock than apparatus where the position calculation is performed in the vehicle mountable device, especially where high values of acceleration and jerk may be experienced. In addition, the bandwidth required for transmitting the relevant signals to a ground or other remotely located base station can be considerably less than that required for the translator system mentioned above.
Preferably, the carrier frequency estimator device comprises a frequency locked loop to reduce data rates and therefore the bandwidth required for transmission between the positioning apparatus and a ground station. Alternatively, an open loop carrier frequency estimation device may be used. A sampler may be arranged to sample the correlator output signals at intervals which are substantially aligned with the data bit periods. This offers simple retrieval of data from the positioning signals.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.